1. Field of the Invention
The present invention relates to a position transducer which detects an amount of travel and moved position of a moving part which is a linearly moving part of a machine tool, industrial robot or the like.
2. Description of the Related Art
Various types of position transducer has been proposed to detect an amount of travel and moved position of a moving part which is a linearly moving part of the machine tool, industrial robot or the like. An example of such position transducers is known from the disclosure in the Japanese Published Unexamined Application No. 10-531835 for example. The position transducer disclosed in this publication includes a scale which develops a magnetic field whose strength and direction change linearly correspondingly to a position of such a moving part, and a magnetic sensor which moves in relation to the scale and detects a magnetic field from the scale at a position to which has moved.
The position transducer proposed in the Japanese Published Unexamined Application No. 10-531835 is constructed as schematically illustrated in FIG. 1.
The position transducer is generally indicated with a reference 100. The position transducer 100 includes a scale 101 and a magnetic sensor 102. The scale 101 consists of a pair of members 101a and 101b formed from a ferrite plastic magnet plate or the like and each having an end face which is oblique at a predetermined angle xcex8 in relation to a direction of movement thereof relative to the magnetic sensor 102. The members 101a and 101b in pair are joined integrally to each other with their respective oblique end faces placed to abut each other. The pair of members 101a and 101b is magnetized to be opposite in polarity to each other in a direction perpendicular to main sides thereof.
The magnetic sensor 102 includes a magnetic core 103 formed from a square ring of a high permeability material such as NIxe2x80x94Fe alloy or amorphous alloy and which forms a closed magnetic circuit, and a pair of detection coils 104 and 105 wound on two longitudinal core pieces, opposite to each other, of the magnetic core 103, respectively. The detection coils 104 and 105 in pair are driven in opposite phases to each other with a high frequency pulsed current to develop magnetic fields in opposite directions.
When the magnetic sensor 102 is at a predetermined distance from the scale 101, the two longitudinal core pieces of the core 103, on which the detection coils 104 and 105 in pair are wound, respectively, are perpendicular to the main sides of the scale 101, a line connecting these two core pieces is perpendicular to a longitudinal center line n of the scale 101 and the mid point between the two core pieces is right above the longitudinal center line n of the scale 101.
In the above geometric relation, the scale 101 and magnetic sensor 102 are installed to a stationary part and a moving part, respectively, of a machine tool, industrial robot or the like, and moved relatively to each other along the longitudinal center line n of the scale 101 as the moving part moves linearly. At each position the magnetic sensor 102 will take along the length of the scale 101, the magnetic sensor 102 will detect a magnetic field developed by the scale 101 and perpendicular to the main sides of the scale 101.
On the assumption that the longitudinal direction of the scale 101, that is, the direction in which the magnetic sensor 102 is moved in relation to the scale 101, is taken as X-direction while a lateral (short-side) direction of the scale 101, perpendicular to the X-direction is taken as Y-direction, and a direction perpendicular to the main sides of the scale 101 is taken as Z-direction, the principle of detection of the position transducer 100 will be described below with reference to FIGS. 2 and 3.
FIG. 2 is a Y-directional sectional view of the scale 101 located at a position where the longitudinal center line n of the scale 101 intersects with a boundary line of the pair of members 101a and 101b forming together the scale 101. As seen from FIG. 3, in the Y-directional section of the scale 101, a magnetic flux "PHgr"z produced in the Z-direction changes linearly in a range of xc2x1W/4 (where W is a Y-directional length of the scale 101) from a Y-directional center of the scale 101, that is to say, the boundary line of the pair of members 101a and 101b forming together the scale 101. Therefore, with a position transducer construction in which when the magnetic sensor 102 is moved in X-directionally in relation to the scale 101, it will have moved substantially in the Y-direction of the scale 101 within a range of xc2x1W/4 from the boundary line of the pair of members 101a and 101b forming together the scale 101, it is possible to detect an X-directionally moved position of the magnetic sensor 102 relative to the scale 101 from the strength of the magnetic flux "PHgr"z produced in the Z-direction.
In the position transducer 100, the X-directional length L1 of the scale 101 is larger than the effective length for detection L2, and an angle xcex8 formed between the moving direction of the magnetic sensor 102 relative to the scale 101 and boundary line of the pair of members 101a and 101b forming together the scale 101 is xcex8=tan xe2x88x921(d/L2)(where d is W/2 or less), so that the magnetic sensor 102 will be moved relatively moved in the X-direction along the longitudinal center line n of the scale 101. Therefore, the Z-directional magnetic flux "PHgr"z at each position the magnetic sensor 102 will take when the latter is moved relatively to the scale 101 will change linearly as in the Y-directional movement of the magnetic sensor 102.
In the position transducer 100, a Z-directional magnetic flux "PHgr"z at each position the magnetic sensor 102 will take when the latter is moved relatively to the scale 101 is detected by the pair of detection coils 104 and 105 driven in opposite phases to each other with a high frequency pulsed current to develop magnetic fields in opposite directions, impedances of these detection coils 104 and 105, which will change correspondingly to an external magnetic field, are converted to voltages, respectively, and a difference between the voltages is determined, to thereby detect a moved position of the magnetic sensor 102 relative to the scale 101.
With the position transducer 100, since a moved position of the magnetic sensor 102 relative to the scale 101 is detected by determining a difference between outputs of the pair of detection coils 104 and 105 driven in opposite phases to each other, it is possible to provide a large output while canceling the influence of electric noises and detect, with an extremely high accuracy, a moved position of the magnetic sensor 102 relative to the scale 101, namely, an amount of travel and moved position of the moving part relative to the stationary part of the machine tool, industrial robot or the like.
The position transducer disclosed in the aforementioned Japanese Published Unexamined Application No. 10-531835 is an excellent one capable of detecting, with an extremely high accuracy of an amount of travel and moved position of a moving part relative to a stationary part of a machine tool, industrial robot or the like. However, the results of experiments conducted on this position transducer proved that if the position transducer is used in an environment where a disturbing magnetic field such as geomagnetism acts strongly, such a disturbing magnetic field and a magnetic field indicative of positional data, developed by the scale 101, cannot be differentiated between them and a change of DC output due to a disturbing magnetic field is superposed on the output from the magnetic sensor 102 and thus no accurate positional data can be provided by the position transducer.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing a position transducer which can be used for position detection in even an environment where a disturbing magnetic field such as geomagnetism acts relatively strongly to provide accurate positional data while suppressing the influence of such a disturbing magnetic field.
The above object can be attained by providing a position transducer including means for developing a magnetic field whose strength and direction change correspondingly to a position of the means, means moved relatively to the magnetic field developing means fro detecting a magnetic field developed by the magnetic field developing means and providing an electrical signal corresponding to the detected magnetic field, and means for detecting a position of the magnetic field detecting means relative to the magnetic field developing means on the basis of the electrical signal provided from the magnetic field developing means; the magnetic field detecting means consisting of two sensor units each formed from a sensor coil wound on a high permeability core and driven in phase with each other with a high frequency, the two sensor units being disposed in different positions in a direction perpendicular to a moving direction of the magnetic field detecting means relative to the magnetic field developing means; and the position detecting means detecting a relative position of the magnetic field developing means to the magnetic field developing means by determining a difference between outputs of the two sensor units.
In the above position transducer, when the magnetic field detecting means consisting of the two sensor units is moved relatively to the magnetic field developing means which develops a magnetic field whose strength and direction change correspondingly to a position of the means, the impedances of the sensor coils of the two sensor units will change corresponding to an amount of travel of the magnetic field detecting means. Since the two sensor units are disposed in the different positions in the direction perpendicular to the moving direction of the magnetic field detecting means relative to the magnetic field developing means, so a magnetic field developed by the magnetic field developing means will have differences in magnetic field from the two sensor units, respectively. Thus, the amount of a change in impedance of the sensor coils, caused by the movement of the magnetic field detecting means relative to the magnetic field developing means, will also be different between the two sensor units. In this position transducer, the two sensor units of the magnetic field detecting means provide electrical signals, respectively, corresponding to the impedances of the sensor coils, respectively, and the position detecting means determines a difference between outputs of the two sensor units, to thereby determine a relative position of the magnetic field detecting means to the magnetic field developing means. Therefore, with either the magnetic field developing means or magnetic field detecting means of the position transducer fixed to a moving part of a machine tool, industrial robot or the like, it is possible to detect an amount of travel and moved position of the moving part.
Also, with this position transducer, since a difference between outputs of the two sensor units disposed in different positions in the direction perpendicular to the moving direction of the magnetic field detecting means relative to the magnetic field developing means and driven in phase with each other with a high frequency, to thereby detecting a relative position of the magnetic field detecting means to the magnetic field developing means, so it is possible to appropriately detect the position while canceling the influence of a disturbing magnetic field such as geomagnetism incident in phase and uniformly upon the sensor units.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.